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R6sum6.-Les calculs relativistes de bandes par la methode des ondes planes orthogonaliskes ont ktk menks A bien aux points de symetrie simple dans la zone rkduite de Brillouin pour plusieurs composCs IV-VI. La structure de bande dans le reste de la zone a et6 deduite par interpolation. A notre connaissance, ce sont les premiers calculs de bandes pour Ies composb IV-V1 qui se basent au depart sur des principes relativistes (contrairement aux calculs non relativistes affines par des corrections relativistes et de couplage spin-orbite). Nos calculs conduisent A des modkles de bandes ayant une rkalitk physique, qui sont suffisamment prkcis pour tenir compte de la plupart des singularitks des spectres expkrimentaux de rkflectivitk. 11 est difficile #interpreter les spectres d'kectroreflectivite sans ambigufte a l'aide de ces modeles de bande mais un certain nombre de recoupements plausibles dans les spectres peuvent &re faits. Des calculs des bandes d'energie et de spectre optique plus Claborks se poursuivent et seront publies ailleurs. Abstract.-Relativistic OPW band calculations have been carried out at key points in the reduced zone for several IV-V1 compounds. The band structure in the remainder of the zone has been filled in with the aid of an interpolation scheme. To our knowledge, these are the first fully relativistic first-principles band calculations for IV-V1 compounds (in contrast to non-relativistic calculations supplemented by relativistic and spin-orbit coupling corrections). Our first-principles band calculations lead to physically realistic energy band models which are sufficiently accurate to account for most of the characteristic features of the experimental reflectivity spectra. It is difficult to interpret the electroreflectivity spectra unambiguously in terms of these band models but a number of plausible spectral assignments can be made. Refined energy band and optical spectrum calculations are in progress and will be reported elsewhere.

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We have developed a method for determining the band structure of crystals which combines the best features of a first-principles approach with the best features of an empirical approach. The first step in our method is a first-principles OPW band calculation based on the free-electron exchange approximation. In the second step, we depart from a purely first-principles approach, and introduce a small empirical correction which brings key features of the calculated energy level scheme into exact (or close) agreement with the most reliably established features of the experimental level scheme. In addition to obtaining reliable energy band models valid over a wide energy range, we obtain the electronic wave functions for a large number of levels, the core and valence electronic charge distributions, and many other "first-principles" quantities. Following a summary of the shortcomings of purely first-principles and purely empirical band calculations, we sketch the essential features of our intermediate treatment. Recent studies of the band structure of diamond, cubic silicon carbide, silicon, and germanium-carried out both by our method and other methods-are then discussed and compared. It is shown how improved band models for these crystals can be generated with the aid of some crucial information about the band structure derived from experiment. Finally, we observe that although the use of a universal exchange potential is an important simplification, and one that has greatly accelerated recent progress in energy band calculations, this approximation is to some extent an oversimplification. I t appears likely that some of the empirical corrections that we now have to introduce in order to bring theory and experiment closer together could be reduced substantially if we could define one exchange potential for s-like electrons, another for p-like electrons, and still another for d-like electrons. This idea of I-dependent exchange potentials represents a middle ground between the Hartree-Fock extreme of different exchange potentials for different orbitals, and the Slater extreme of the same exchange potential for all orbitals.